Income Taxes - Reconciliation of U.S. Federal Statutory Rate to Arconic's Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
U.S. federal statutory rate	35.00%	35.00%	35.00%
Taxes on foreign operations	(7.50%)	(10.20%)	2.50%
Federal benefit of state tax	3.10%	0.40%	0.30%
Permanent differences on restructuring and other charges and asset disposals	(167.40%)	(107.80%)	3.60%
Non-deductible acquisition costs	0.30%	8.40%	7.10%
Statutory tax rate and law changes	52.50%	(15.70%)	(1.00%)
Tax holidays	(3.00%)	(0.80%)	(3.90%)
Tax credits	(0.70%)	(1.20%)	(2.80%)
Changes in valuation allowances	137.90%	426.80%	145.80%
Impairment of goodwill	53.50%	0.00%	4.80%
Company-owned life insurance/split-dollar net premiums	(0.00%)	23.00%	(3.00%)
Changes in uncertain tax positions	10.10%	2.10%	(2.20%)
Other	1.90%	(3.50%)	(1.00%)
Effective tax rate	115.70%	356.50%	185.20%